A process for obtaining high strength and high initial modulus fibers which comprises gel spinning an ultra-high-molecular-weight polymer solution and stretching the resulting unstretched fiber at a high stretch ratio has recently been developed and is attrating attention.
Polyethylene fibers obtained by the gel spinning technique have higher strength and higher initial modulus than aramide fibers and are now being brought to the commercial stage. Polyethylene fibers, however, have poor heat resistance due to their low melting point, which has been a bar to broadening of their application as industrial materials.
Of general purpose fibers, PVA fibers are excellent in strength and initial modulus and also have a melting point of from 230.degree. to 240.degree. C. or even higher and are thus very useful as industrial materials.
Many proposals have hitherto been made in an attempt to improve the strength and initial modulus of PVA fibers. For example, U.S. Pat. No. 4,440,711 discloses a process comprising gel spinning a glycerin solution of an ultra high polymerization degree PVA having a molecular weight of 500,000 or more and stretching the resulting unstretched fibers at a ratio of about 20. However, not only is such an ultra high polymerization degree PVA difficult to commercially produce, but also the spinning solution thereof has too high a viscosity to be easily spun. Since the polymer concentration of the spinning solution has to be reduced to improve spinnability, productivity of this process is low.
U.S. patent application Ser. Nos. 06/842,577 and 06/882,049 (both abandoned) (corresponding to JP-A-62-282015 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")) disclose a process comprising gel spinning a solution of PVA having a molecular weight of from 100,000 to 500,000 (corresponding to a polymerization degree of from 1400 to 7000) and stretching the resulting unstretched fibers to obtain filaments of 16 to 50 denier. However, while fiber strength is reduced with an increase of fineness as revealed in Table III therein, there is no disclosure as to how to prevent such a strength reduction when the fineness becomes higher. Moreover, the maximum fineness of the filaments obtained by the disclosed process is about 50 denier, and filaments having such a maximum fineness have a strength of only 9.9 g/d and an initial modulus of only 293 g/d. High strength and high initial modulus PVA monofilament yarns as proposed by the present invention cannot be obtained by this process.
U.S. Pat. No. 4,765,937 discloses a process for producing high strength and high initial modulus fibers by spinning a PVA solution in a mixed solvent of water and an organic solvent and stretching the unstretched fibers, followed by heat setting. This patent is silent as to fineness. It is impossible to estimate the fineness only from the conditions given in the working examples of this patent, and monofilaments having a fineness of 100 denier or more cannot be expected from this process.
U.S. Pat. No. 4,971,861 (corresponding to JP-A-63-165509) discloses a process for producing high strength and high initial modulus fibers by spinning a solution of PVA in a specific solvent and stretching the resulting fibers in multiple stages so as to provide thereto an overall stretch ratio of at least 15. From the disclosure in the working example thereof that unstretched fibers of 5300 d/100 f were stretched at a ratio of from 14.2 to 26.6, it is clear that a single yarn had a fineness of from 4.0 to 7.5 denier. Therefore, this process provides fibers having a fineness of 10 denier at the highest, and is incapable of producing high strength and high initial modulus monofilament yarns having a fineness of 100 denier or more.
Further, U.S. Pat. Nos. 4,698,194 and 4,603,083, JP-A-61-108711 and JP-A-61-215711 disclose a process in which a spinning dope of PVA having a polymerization degree of 1500 or more in dimethyl sulfoxide (hereinafter abbreviated as DMSO) or glycerin is subjected to dry-wet spinning or gel spinning at a spinning draft (the ratio of the take-off speed to the spinning linear speed of the fiber spun from the nozzle) of from 0.05 to 1.0 and the resulting unstretched fiber is stretched at a rate of at least 20. According to the disclosed process, in order to make the spinning draft 1 or less by increasing the spinning linear speed, the spinning nozzle used must have an extremely small diameter such as 0.08 mm. Such a nozzle is not only difficult to produce but is capable of providing only fine fibers whose single yarn size is 10 denier or less. Actually, as described in JP-A-61-215711, it is considered essential for the single yarn to have a fineness not exceeding 5 denier in order to obtain high strength and high initial modulus PVA fibers. Thus, high strength and high initial modulus PVA fibers having a single yarn fineness of 5 denier or more which are made from PVA whose polymerization degree is within a commercially available range are currently unknown.
The inventors of the present invention disclosed in Japanese Patent Application No. 1-122030 PVA fibers having a tensile strength of 13 g/d or more, an initial modulus of 300 g/d or more, and a single yarn fineness of 10 denier or more and less than 100 denier and a process for producing the same. The fibers obtained by this process also unavoidably undergo a reduction in strength and initial modulus with an increase in single yarn fineness. If the single yarn fineness exceeded 100 denier, the fiber strength was about 9 g/d at the highest.
On the other hand, a process for producing a thick monofilament yarn by closely adhering a plurality of filaments spun from a plurality of spinning orifices and uniting them into one body is disclosed in U.S. Pat. No. 2,891,277 and British Patent 838,141. The disclosed process is directed to an improvement of the profile of polystyrene fibers, polyester fibers, etc., but not to an improvement of the mechanical properties of monofilament yarns, such as tensile strength and initial modulus.
Cases are often met in which filaments spun from orifices by general melt spinning are adhered to each other under some spinning conditions, and such a phenomenon is regarded as unfavorable because the resulting fibers would have impaired physical properties. To the contrary, it is an improvement of a fiber profile that has been expected by intentionally inducing this phenomenon.
As mentioned above, none of the so far proposed processes for producing PVA fibers having high strength and high initial modulus has achieved high strength and high initial modulus in monofilament yarns having a fineness of not less than 100 denier. It has been a generally accepted belief that strength and initial modulus are reduced as single yarn fineness is increased.